"No Operating System Found" is an error from your BIOS. This tells us your hard drive is no longer bootable for whatever reason. Just so you know - this isn't a Windows problem - this is a drive, file system or boot record problem.
Make sure you remove any floppy disks that may be inserted into your floppy drive. Ensure you've connected the drive properly and master & slave settings are properly set. Double check and triple check both the power and data cables connected to your drive at both ends. Lastly, enter the CMOS/BIOS setup and ensure the boot device order is correct.